In recent years, the advent of the inexpensive smoke alarm has provided consumers with a simple means of early fire detection. This has resulted in the saving of many lives and in reductions of property damage where fires have occurred.
The present standard for smoke alarms specifies a failure rate of 3.5 failures per million operating hours. While this seems to be a low rate, the huge numbers of devices in service means that a small number of them have failed. There is evidence that among devices more than 10 years old, 20 percent may no longer be functioning, and after 20 years more than 50 percent are likely to have failed. Thus, many thousands of consumers may falsely think they will be warned if smoke appears.
In the typical photoelectric smoke alarm, a darkened volume in communication with the surrounding air is illuminated from one side by a beam of light from a light emitting diode or the like. If smoke is present within the space, some of the light will be scattered perpendicular to the path of the beam. A photodetector is mounted to the side of the darkened space and pointed so that it will detect the scattered light but not the light beam. The photodetector detects light only if particles suspended in the air such as particles of smoke scatter a portion of the light beam. If there is no smoke, there is no scattered light, and hence no signal from the photodetector. This has the advantage that the photodetector and associated circuitry can be operated at high gain, since any signal would be due to smoke. On the other hand there may be no signal because the light source has failed. The photodetector cannot measure the difference. Further, the associated circuitry is unable to tell if the absence of signal may be due to a failed photodetector. Thus, this commonly used method can have several undetectable failure modes which result in failure to detect smoke.